Field of the Invention
The field of the invention is that of Doppler lidar systems installed onboard aircraft. These systems allow airspeed to be measured at a certain distance from said aircraft.
Description of the Related Art
This technique is based on the measurement of the Doppler shift induced by particles present in the atmosphere in the backscattered laser wave. The use of a plurality of laser beams or a beam-scanning system allows the three components of the airspeed vector to be determined in the entire flight envelope of the carrier.
These systems require the use of an optical porthole that is almost conformal with the skin of the carrier. This porthole allows the laser beam to pass into the atmosphere without altering its optical and geometric properties and without attenuating the emitted power.
On account of the operating conditions of aircraft, a suitable heating system is indispensable in order to prevent icing of the porthole when atmospheric temperature conditions combine with the presence of water. Icing has the effect of degrading the optical performance of the porthole and therefore of decreasing the precision of the measurement of airspeed. Under certain severe icing conditions, the measurement may even be completely lost.
To prevent icing of the porthole, a number of technical solutions exist. A first solution consists in using conduction to heat the porthole. Electrical resistors are placed making contact with the supporting structure of the porthole and allow the required thermal power to be generated. A second solution consists in depositing a resistive film on the surface or in the thickness of the porthole. This second solution is, for example, used to deice aircraft windscreens. A thin resistive film is then deposited between two sheets forming the windscreen.
The main disadvantages of these two solutions are the following. The first solution generates substantial thermal stresses due to the localized injection of thermal power. These stresses deform the wavefront of the transmitted optical wave on emission and reception. This deformation may lead to a substantial decrease in the received signal level and degrade measurement precision or even prevent the measurement if the signal-to-noise ratio is too greatly degraded. Furthermore, it is not envisageable to use this technique to deice large optical areas. Lastly, the heating of the frame of the porthole represents a substantial source of power consumption due to the dissipation over a larger area of the thermal power delivered. One solution allowing these various drawbacks to be partially mitigated is the use of a porthole made of sapphire. This material has a very high thermal conductivity. If the porthole has a sufficiently large area, the heating resistors may be adhesively bonded directly to the optically unuseful surface of the sapphire porthole. However, employing sapphire has a drawback: it is birefringent. If polarized light is employed, the useful signal level may decrease if this birefringence is not correctly accounted for.
The main drawback of the second solution is a loss of transmission due to the reflectivity of the heated films.